I have an Otterbox Defender with a built in screen protector for my Mini 6 and so far it’s doing a great job.I use a hard shell case that has a built in screen protector. Saved me from many drops!
I suspect though, over time, that the various gaps in the case will allow a fair amount of dust and grime to find its way under the protector which will require the case to be dismantled in order to clean the screen.
No biggie, but worth thinking about.